Although the invention is broadly applicable as above indicated, the problem to which it is addressed is especially well illustrated by considering the field of radiation therapy. A principal goal of such therapy is to deliver a radiation dose appropriate to achieve the purpose of treatment (e.g. tumor eradication, palliation), while simultaneously minimizing dosage to surrounding normal (healthy) tissues, reducing the likelihood of clinically significant damage to these tissues. These objectives may be appreciated by referring to the schematic depiction of FIG. 1, which illustrates the ICRU definitions for appropriate target volumes for a patient undergoing radiation therapy. (See Landberg, T. Chavaudra, J.; Dobbs, J.; Hanks, G.; Johansson, K.; Moller, T.; Purdy, J.; "Prescribing, Recording and Reporting Photon Beam Therapy". International Commission on Radiation Units and Measurements, 50, 1993. The Gross Tumor Volume (GTV) is the gross palpable or visible/demonstrable extent and location of malignant growth. The Clinical Target Volume (CTV) is a tissue volume that contains a demonstrable GTV and/or subclinical microscopic malignant disease, which has to be eliminated. This volume thus has to be treated adequately in order to achieve the aim of therapy, cure or palliation. The Planning Target Volume (PTV) is a geometrical concept, and it is defined to select appropriate beam sizes and beam arrangements, taking into consideration the net effect of all the possible geometric variations, in order to ensure that the prescribed dose is actually absorbed in the CTV. The Treated Volume is the volume enclosed by an isodose surface, selected and specified by the radiation oncologist as being appropriate to achieve the purpose of treatment (e.g. tumor eradication, palliation). The gross target volume (GTV) and clinical target volume (CTV) thus contain tissues to be treated, while the planning target volume (PTV) places a margin around the CTV to account for patient movement and uncertainties in treatment set up.
In the last several years, conformal therapy techniques are being used with increasing frequency. These techniques combine good patient immobilization to minimize the PTV margin around the CTV coupled with the use of multiple, non coplanar beams to reduce treated volume margin beyond the PTV. The ultimate goal of conformal therapy is to deliver a treatment in which the CTV, PTV and treated volumes are identical. The use of intensity modulated radiotherapy (IMRT) and multiple, non coplanar beams has substantially improved the conformation of the treated volume to the PTV.
In the thorax and abdomen, the PTV margin beyond the CTV remains relatively large. A large contributory factor is the organ motion due to respiration. Motion of the heart, kidney, liver, pancreas, and spleen may be several centimeters, requiring a large PTV. In diagnostic imaging, organ motion has been recognized as a significant cause of image blurring. Several techniques may be used to reduce respiratory organ motion. Retrospective or prospective image correction techniques such as navigator echo imaging work well for MRI, but are not applicable to radiation therapy. Breath holding has been used with success for spiral CT scanner image acquisition but is not practical for radiation therapy, because the beam on time is typically too long for most patients to hold their breath. In lithotripsy, respiratory induced kidney motion inhibits accurate localization.
In radiation therapy, the potential reduction in the PTV margin, accomplished by minimizing the effects of organ motion due to respiration, may be as great as the reduction in the treated volume margin gained by using conformal therapy techniques. Yet, methods to reduce organ motion in radiation therapy have been limited and typically have employed impedance plethysmography or pneumotachometry, measuring changes in chest or abdomen position or pressure, usually by means of a sensor such as a belt attached to the patient. These devices may have calibration problems caused by variation of the tightness of the belt between treatments or slippage that occurs during treatment. If the device is in the beam, radiation damage to the device or loss of skin sparing may occur. In radiation therapy, the challenge is not simply to freeze respiratory motion for a single session, as is the case in diagnostic imaging, but to do so reproducing the diaphragm position between consecutive respiratory cycles, radiation beams and treatment days. The procedure employed must ensure reproducibility of organ position not only during treatment but for diagnostic image acquisition used in treatment planning as well.